I had some spare time so I hunted down some ebooks that are available. Not sure if this will help you but I'm sure it will help someone. For the Google Book links, there is a download PDF link at the far right near top. Just click that and the "book" will be yours.
Angle, Glenn - Airplane Engine Encyclopedia (1921):
Airplane engine encyclopedia: an ... - Google Books
Bacon, John - Elements of Aviation Engines (1918 ):
Elements of aviation engines - Google Books
Burls, George - Aero Engines (1915):
Aero engines: with a general ... - Google Books
Chadwick, John - Aviation Engines (1919):
Aviation engines - Google Books
Kean, Francis - Aeronuatical Engines (1916):
Aeronautical engines: a critical ... - Google Books
Marks, Lionel - The Airplane Engine (1922):
The airplane engine - Google Books
Page, Victor - Aviation Engines (1918 ):
Aviation engines: design ... - Google Books
Pollard, Harold - Aero Engines, Magnetos, and Carburetors (1918 ):
Aero engines, magnetos and carburetors - Google Books
Sherbondy, E. H. and G. Wardrop - Textbook of Aero Engines (1920):
Textbook of aero engines - Google Books
Smithsonian Annals of Flight (10 volumes, 6 of which deal with aircraft engines):
Smithsonian Annals of Flight
Wilkinson, Paul - Diesel Aviation Engines (1940) in six parts:
Diesels
Others that may be of interest:
Colvin, Fred - Aircraft Mechanics Handbook (1918 ):
Aircraft Mechanics Handbook - Google Books
Flight Magazine Archives:
Aviation History - Browse the History of Flight from 1909
Moss, Sanford - The Gas Turbine (1903):
The gas turbine: an "internal ... - Google Books
NASA/NACA Technical Reports:
NASA Technical Reports Server
Various engine books not so aviation related:
DigitalBookIndex: ENGINEERING: ENGINES / MOTORS: INTERNAL COMBUSTION (e-Books, eTexts, On-Line Books, eDocuments)
Enjoy,
WJP